Oil wells are formed by deposits of oil or gases, or of a mixture of oil and gas, of solid residues and water, enclosed in rock, sand or clay formations, hereinafter referred to as “formations”, whether consolidated or not, having high or low permeability. These wells may be of different levels of depth, from superficial or shallow to medium to deep wells. Once the well has been drilled and depending on the permeability of the environment, the extraction of the oil or gas begins. The oil or gas comes out of the formation, naturally or with the aid of external pressures, due to the permeability created by natural or artificial fractures in the formation until it reaches the surface, usually by means of metallic tubing (“wellbore”).
Depending on the permeability, depth and on the pressures on the formation, after the well is drilled the oil or gas may not come out. On the other hand, even in high-permeability wells, from which the oil or gas comes out relatively easily, the continuous passage of the gas through pores and/or natural fractures drags solid residues and impurities which gradually block the fractures and consequently also block the oil or gas flow.
In order to solve the problem described above, techniques were developed to stimulate the wells so as to allow the oil or gas to flow properly. This stimulation technique, known as “hydraulic fracturing”, consists of obtaining formation fractures by injecting fluids, usually brine. Once the fractures are formed, the same system is used, that is, the injection of fluids such as brine-based fluids containing solid particles with a wide range of mechanical strength. These particles are introduced into the fractures and when the fracturing pressures are removed, said particles present in the fractures prevent the latter from closing. The channels (fractures) obtained and filled with said particles provide a permeable means through which the oil or gas will be extracted from the well.
The aforementioned solid particles, known as “proppants” or propping agents, can be of different types, and are usually made of materials such as natural sand, resin-coated sand, spherical ceramic proppants, whether resin-coated or not, ceramic proppants for the control of the so-called flow back phenomenon, among others.
The proppants conventionally used for hydraulic fracturing are spherical ceramic proppants, such as the ones described, for example, in patent documents BR 9501449-7, BR C19501449-7 and BR 9501450-0, wherein sphericity is a requirement to obtain a fracture with maximum permeability. In the traditional hydraulic fracturing operation with natural or synthetic proppants, particle sizes predominantly vary in the range of from 8/12 to 35/70, and more predominantly in the ranges of 16/20, 16/30, 18/40 and 20/40. Minimum amounts are used outside these ranges, and all the grades are composed of spherical particles. The pelletizing processes (shaping particles into spheres) known and used to obtain these particle size ranges produce very small amounts of particles smaller than 40 mesh, do not contemplate the production of fine particles and are not able to produce them in an economically viable manner.
There are, however, applications for which the use of proppants with more abrasive characteristics than the spherical proppants currently existing and cited herein is desirable. For the particles to be more abrasive than the ones found in spherical proppants they should be angular shaped.
The conventional angular abrasive material for use in fracturing operations in oil wells is the one obtained from natural sand or natural sand-based materials, with a particle size of 100 mesh. The proppants produced with natural sand particles, however, have low mechanical strength and when used in fractures subject to significantly high confinement pressures, usually ≧4000 psi, they cannot resist and break into smaller particles, drastically reducing the permeability of the environment and increasing the flow back in the fracture stimulated with this type of proppant, with a consequent loss in productivity.
Document BR 0301036-8 defines a proppant comprising a mixture of a spherical ceramic material with an angular material for the specific purpose of controlling or preventing the flow back phenomenon, that is, the reflux of the proppant when the well goes into operation. Therefore, this is a proppant wherein the addition of angular particles in smaller proportions to a traditional spherical proppant to control, stop and obstruct the rolling motion of spherical proppants, thus preventing flow back. This proppant is produced with particles, whether angular or spherical, of up to 40 mesh in size, and contains no particle smaller than 40 mesh.
Patent document U.S. Pat. No. 6,059,034 also discloses a proppant comprising a mixture of a spherical ceramic material with a material having definite curves and an elliptic format, said angular material comprising a non-refractory, low-hardness, organic deformable polymer, which cannot stand temperatures above about 200° C.
Patent document BR 9700995-4 defines a process for the preparation of a bauxite abrasive material for use in surface finish jetting. The process described in said document comprises the steps of drying and grinding raw bauxite to a particle size smaller than 45 μm, followed by the pelletizing of the dried and ground material and subsequent sintering and grinding of the resulting sintered material to obtain angular particles with the desired characteristics for the finish jetting processes.
The aim of the present invention is to provide an angular proppant having abrasive characteristics, high mechanical strength, and which is thermally and chemically stable, in very fine fractions having high mechanical strength so that will not break easily during the fracturing of oil and gas wells.